Samsung
Set Date Manually auf Samsung
Samsung phones normally take the date from the mobile network or from a connected Wi-Fi source. This keeps the calendar, apps and security certificates in sync without any manual input.
If you need a different date - for testing, after a SIM change, or because the network signal is unreliable - the date field stays locked as long as automatic sync is active. You have to switch that function off first before the phone lets you pick a day, month and year on your own.
Menu path to set the date manually
Open Tap on Settings ➔ Tap on General management ➔ Tap on Date and time ➔ Turn Off Automatic date and time. Once the toggle is off, tap Tap on Set date, choose the date you need and confirm with Set the Date and tap Done.
Keep in mind that a manually set date can drift out of sync over time, since the phone no longer corrects itself against the network. If you only had a brief signal issue, turning automatic date and time back on afterward is usually the safer long-term option.